The biochemical oxygen demand (BOD) test is a crucial environmental index to determine the relative oxygen requirements of waste waters, effluents and polluted water. It refers to the quantity of oxygen required by bacteria and other microorganisms in the biochemical degradation and transformation of organic matter under aerobic conditions. The test is also interpreted as a measure of the concentration of organic material that can serve as a substrate to support the growth of microorganisms.
The BOD test, as used for assessing the efficiency of waste water treatment, is intended to be a measure of carbonaceous oxygen demand as well as nitrogenous demand. This is known as ultimate BOD. Since ammonia is usually present in some waste waters, nitrification inhibitors must be used to suppress the exertion of nitrogenous oxygen demand. Carbonaceous oxygen demand is called first-stage BOD and nitrogenous oxygen demand is called second stage BOD. In waste waters which do not contain the nitrogenous matter, the oxygen consumed by heterotrophic microorganisms is the carbonaceous BOD. There are number of methods to estimate the water pollution potential. The chemical oxygen demand (COD) test was developed because a BOD5 test requires 5 days for completion and therefore is not suitable either for real-time evaluation of the efficiency of waste water treatment or for operational control of the treatment processes. The total organic carbon (TOC), dissolved organic carbon (DOC), the UV spectrophotometric absorption at 254 nm for dissolved organic material and volatile suspended solids for particulate organic material were developed as alternative methods for measuring the strength of waste water on the basis of an assumption that the primary purpose of biological treatment is to reduce the concentration of organic material in the waste water.
The conventional BOD test has certain benefits such as being a universal method of measuring most wastewater samples, and furthermore, no expensive equipment is needed. It has, however, the limitation of being time consuming, and consequently it is not suitable for on-line process monitoring. Thus, it is necessary to develop a measurement method that could circumvent the weaknesses of the conventional method. The fast, portable and cost effective methods for environmental monitoring have stimulated the development of a variety of field analytical tools such as biosensors. Biosensors are devices that transduce a selective biochemical response to a measurable signal. Several biosensor methods for BOD measurement have been developed. The first report of BOD biosensor was published by Karube et al. in 1977. After that, several kinds of microbial BOD sensors have been developed and various modifications have been carried out. Most of the above reported BOD sensors consisted of a synthetic membrane with single or a random combination of immobilized microorganisms serving as biocatalyst. A rapid and reliable BOD sensor should aim at being highly capable of analyzing a sample of complex constituents with relatively low selectivity but high sensitivity. Thus, the sensor can respond to all kinds of biodegradable organic solutes in the samples. It is also important that the sensor should give results comparable to those obtained using the conventional BOD method.
Most of previously reported BOD sensors are bio-film type whole-cell-based microbial sensors, which rely on measuring the bacterial respiration rate in close proximity to a suitable transducer. A common feature of these sensors is that they consist of a microbial film sandwiched between a porous cellulose membrane and a permeable membrane as the biological recognition element. This microbial film is immobilised microbial populations that can bio-oxidize the organic substrate to be quantified. The response is usually a change in concentration of dissolved oxygen (DO) or other phenomena such as light emission. A physical transducer is used to monitor this process. The result is a change in an electrical or optical signal. The signal is amplified and correlated to the content of biodegradable material measured.
A biofilm sensor for BOD consists of a DO probe including the oxygen-permeable membrane, with another membrane containing bacteria (the biofilm) between the membrane and the sample. Organic material diffuses into the biofilm where the bacteria act on it, causing a drop in oxygen levels that is measured by the probe. The probe is calibrated by setting a baseline level (‘zeroing’) with a solution containing no organic material, followed by measuring the steady-state response in a solution of known BOD—normally 150 mg glucose plus 150 mg glutamic acid in 1 litre of distilled water taken as approximately 200 mg l-1 BOD. The response of unknown samples is proportioned to the standard to obtain the BOD result. The time quoted to reach a steady reading varies from under 10 minutes to 30 minutes.
A related technique utilizing fluidized bed biology (in place of the biofilm) with DO probes has been incorporated into a commercially available instrument.
Another method uses a biofilm as in the previous method but, instead of waiting for a steady-state reading, it measures the rate of change of oxygen uptake by the bacteria, obtaining results in approximately 1 minute.
The methods are similar to a BOD test, using bacterial activity for their measurement and consequently responding strongly to easily biodegradable material. Many researchers use a monoculture; how well this approximates a mixed culture a response will depend mainly on the sample. Some researchers have had trouble obtaining reproducible results from sensors that used mixed microorganisms from activated sludge.
Apart from the effects of dilution on BOD, the biofilm sensors cannot include degradation of organic particulate matter to any significant extent since the bacteria are immobilized within the sensor system. It has been shown, however, that the biofilms can be acclimatized to different substrates (wastewater as well as pure compounds) to give improved responses.
Biofilm techniques allow rapid process control; they are most likely to find application in monitoring a consistent organic process waste.